1. Field of the Invention
The present invention relates to a hydraulic shock absorber which absorbs mechanical impact caused in a process of stopping a moving object with the aid of the flow resistance of oil which flows from a piston chamber in a cylinder housing.
2. Description of the Related Art
Hydraulic shock absorbers which control damping capacity to impact or reduce the peak value of impact acceleration as much as possible are disclosed, for example, in Patent Literatures, Japanese Unexamined Utility Model Registration Application Publication No. 62-140241 and Japanese Unexamined Patent Application Publication No. 2006-250309. The shock absorber disclosed in JP62-140241U has a cylinder housing and a piston chamber formed in the cylinder housing, and the piston chamber is filled with oil and formed in a taper shape which narrows so as to form a linear or quadratically curved structure in a direction of the movement of a piston. The shock absorber disclosed in JP2006-250309A has a piston chamber with an inner surface formed in a tapered structure which linearly narrows in a direction of the movement of a piston in a taper ratio ranging from 1/50 to 1/130.
In the shock absorber disclosed in JP62-140241U, since the size of an orifice formed in a gap between the cylinder housing and the piston is decreased in conjunction with the movement of the piston, a capacity to damp impact is small at the start of the damping. In the case where a rod moves to decrease the size of the orifice, an energy absorption rate increases, and the speed of a moving object to be stopped by damping decreases. In the shock absorber disclosed in JP2006-250309A, an experiment is performed with specific experimental equipment under certain conditions, and the result of the experiment indicates that the peak of impact acceleration is small in a taper ratio ranging from 1/50 to 1/130 and that such a range is therefore effective for the damping of impact.
The orifice formed between the cylinder housing and the piston desirably contributes to the following processes: in the early stage in which a moving object to be stopped by damping collides against the tip of the rod and then the stop by damping starts, impact strength is damped while the orifice area is secured in large, thereby decreasing collision noise and dust emission; in the subsequent middle stage, the amount of energy absorption is enhanced to decelerate the object to an extent in which the moving object is prevented from rebounding in the final stage; and the object is stopped by damping in the final stage. Unfortunately, the shock absorbers disclosed in JP62-140241U and JP2006-250309A cannot satisfy this desirable requirement.
In particular, although the shock absorber disclosed in JP62-140241U, especially, a shock absorber having a quadratically curved taper structure illustrated in FIG. 1 of JP62-140241U, can damp impact strength and decrease collision noise and dust emission to some extent in the early stage in which a moving object starts to be stopped by damping, energy is insufficiently absorbed in the middle stage. Since large resistance is rapidly applied in the vicinity of the terminal of a stroke, the moving object cannot be sufficiently prevented from rebounding at a stopping place. In particular, increase in the speed at which the moving object collides against the rod causes the moving object to rebound at the stopping place. The sufficient stop by damping cannot be provided at all.
This problem is explained with reference to the result of an experiment made by the inventors. FIG. 2(A) illustrates the relationship of the inner diameter of a piston chamber and a position of a piston stroke between a case in which a piston chamber has a curved inner surface a as in a representative embodiment of the present invention and a case in which a piston chamber has a tapered inner surface b as in the shock absorber disclosed in JP 62-140241U or JP2006-250309A. FIG. 2(B) schematically illustrates the relationship of the stroke position and resistance applied to a moving object by a rod in the individual cases, the relationship being indicated by curves a′ and b′, respectively.
As is obvious from FIGS. 2(A) and (B), in a known hydraulic shock absorber having a piston chamber with a tapered structure, since an orifice starts to narrow even though the moving object rapidly collides against the rod in the early stage in which the moving object starts to be stopped by damping, resistance applied to the moving object by a rod, namely, energy absorption by the shock absorber, does not sufficiently contribute to the damping of impact strength in the early stage as indicated by the curve b′ in FIG. 2(B) as compared with the case in which the size of an orifice is constant (for example, the case illustrated in FIG. 3 in JP62-140241U). Thus, satisfactory decrease in collision noise and dust emission is not provided. As illustrated in FIG. 2(a) in JP62-140241U and FIG. 2(B) in which the curve b′ indicates large resistance that causes rebound in the vicinity of the terminal of the stroke immediately before the stop of the moving object by damping, the energy absorption in the middle stage after the early stage is obviously insufficient.
Shock absorbers are required to have capability to stop significantly various types of moving objects by damping, whereas it is difficult to produce a piston chamber which enables various types of stop by damping. The above type of stop by damping is employed in spite of insufficient control characteristics because the piston chamber is formed so as to support as various types of stop by damping as possible and so as to have high moldability.
However, a shock absorber is often demanded, which has good performance and characteristics at some degree of sacrifice of moldability. A shock absorber should be therefore provided, which has some moldability and has performance satisfying user demands as much as possible.